Metering pumps are known that comprise a pump body defining a cylindrical pump chamber having an admission orifice and a delivery orifice situated at opposite ends of the chamber. A piston associated with a drive rod is mounted inside the pump chamber to slide with reciprocating motion between the admission orifice and the delivery orifice. The piston has a pump orifice passing therethrough, which orifice is alternately opened and closed by a non-return valve.
With pumps of that kind, the fluid to be packaged comes into contact with both faces of the piston, thereby solving some of the liquid contamination problems that arise with conventional positive displacement pumps in which the liquid to be metered out comes into contact with one side only of the piston (the admission and delivery orifices of the pump chamber then both being situated at the same end of the chamber).
However, in existing pumps, the drive rod passes through the wall of the pump body, thus making it necessary to provide sealing between said wall and the drive rod. This sealing gives rise to difficult problems, particularly when the fluid to be packaged is abrasive or chemically aggressive, since the reciprocating motion of the drive rod runs the risk of damaging the gaskets quickly. Major problems also arise when fluids need to be packaged in sterile manner because of the pollution that might be conveyed by the drive rod in its reciprocating motion: it is then necessary to provide complex devices making use of sterile fluid or vapor barriers, or indeed membranes which are fragile.